Two different panels/circuits sharing the same conduit

I have a situation where I would like to use the same conduit run to carry wire from a main panel and a sub-panel. The answer to the question why is that in a power outage the sub-panel is generator protected. There is a circuit on the sub-panel which doesn't need generator protection and there is a circuit on the main panel that does. Given the two panels are 70 feet from each other and the two circuits outlets in question overlap by 20 feet - I just want to run each one in the same conduit past each other effectively, but mentally I seem to think this isn't a good idea - i don't think there is a technical issue here, but can't find the code related to this ... thoughts?

It sounds like this is a feed between the main panel and a sub panel. A set of wires feed power from the main panel to the sub panel, the sub panel also has generator backup power provided to it, one circuit in the sub panel does not need backup power but another circuit in the main panel does. The op wants to know if he can feed power between the sub panel and the main panel to provide backup power to one circuit in the main panel through the same conduit that feeds the sub panel.

Thank You!

Thanks for all the input guys! It isn't in the main raceway from panel a to panel b, merely some 1/2 inch emt off the panel. Don't think any derating is required given only 6 ccc are in the emt (2 hot/neutral pairs and a hot/switch pair - so you don't have to have a neutral for every hot in this section of emt). Thanks again all is good!